Anastasius III Bibliothecarius (circa 810- 879) was an antipope of the Roman Catholic church, during the year of 855. He was known for his great learning, in an age where violence was abundant and scholars were in short supply. He was fluent in Latin and Greek, doing translations to Latin and also writing original works.

He was placed as pope when the ahderents of Louis II, Holy Roman Emperor managed to have the election of Pope Benedict III disavowed, only to later see Benedict be consecrated as pope anyway, under popular pressure. Pressure from the holy roman emperor resulted in Anastasius being treated leniently, after his stint as an antipope.

Anastasius was called "bibliothecarius" because he later served as the librarian of the Church, being named there around 867 by pope Adrian II at a time when this was an influential position in the papal court. He also held this position under Pope John VIII, Adrian's successor, probably till 879.

Anastasius undertook diplomatic work for the papal court, both in Italy and in faraway places. He was ambassador for the holy roman emperor Louis II to the Constantinople court of Basil I, the Byzantine emperor, who had succeeded emperor Michael III, the drunkard, by having him assassinated.

While in Rome, Anastasius met the orthodox monks Saint Cyril and Saint Methodius and supported their christianizing efforts among the slavs. This was before the great East-West Schism, which split the catholic church in two, so his support must not be taken as a brave and irresponsible act.

According to some scholars Anastasius the antipope and Anastasius the librarian are two different persons, while according to others they are one and the same.